Hydrogen Effect on Nitrogenase (C2H2Reduction) Response to Oxygen inBradyrhizobium japonicum-Phaseoleae Symbioses

Abstract
The influence of hydrogenase in Bradyrizobum-Phaseoleae symbioses was studied ex-planta and in-planra in soybean (Glycine max) and cowpea (Vigna unguiculata). The hydrogenase was activated by the addition of hydrogen in the incubation gas phase which modified the response of nitrogenase activity of Hup+ (hydrogen uptake positive) symbiosis to the external oxygen partial pressure. For bacteroids the hydrogenase expression increased nitrogenase activity at supraoptimal pO2, acting possibly as a respiratory protection of nitrogenase. However, at suboptimal pO2, nitrogenase activity of Hup+ bacteroids decreased with hydrogen, a phenomenon attributed to the lower efficiency of ATP synthesis from hydrogen than from carbon substrates oxidation. For undisturbed nodules, the hydrogenase expression in soybean increased the optimal pO2 for ARA (COP), from 35.3 to 40.3 kPa O2, and the ARA at supraoptimal pO2; at suboptimal PO2 there was a negative effect of hydrogenase on ARA, although this inhibition was less than on bacteroids and was not detected if plants were grown at 15 °C rather than 20 °C root temperature. No H2 effect was detected on cowpea nodules. The results on soybean nodules are consistent with the concept that symbiotic nitrogen fixation is oxygen-limited and that hydrogenase activity has no beneficial effect on nitrogen fixation in O2 limitation.

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